Wonderful! I was wondering about something to do just that (see
https://www.mobileread.com/forums/showthread.php?t=69837).
Your script works on my Mac! I will tinker with it in depth shortly, but it looks like just what I want to amend CSS from ePub titles that I have purchased, but which don't render nicely on the PRS-600 or iPhone. Brilliant stuff.
Many thanks.